OK. Found the problem.
The number of audio formats that the OpenOffice media player, actually JMF, can play is so limited that unless the media file attached to the slide show is in one of those few formats, it you won't get a sound. Simple as that.
There is a way to expand the number of formats JMF can play, per the document bgeddy was kind enough to provide a link to, but after over two hours of trying to download and compile the file, forget it. More trouble than it is worth. I have XP running in VIrtualBox and will just play the pps files through, ugh, ms-powerpoint.
It is moments like this that make me think of giving up and bowing to the "evil empire" in Redmond, but I get over it.
BTW, have you read about "XP Mode" for windows 7? Seems mickeysoft can't even get that done correctly.